The present invention relates to mixers for use in wireless transceivers, specifically a multi-phase mixer.
The field of wireless technology is currently undergoing a revolution, and is experiencing exponential growth. Cell phones, once considered a novelty and referred to as xe2x80x9ccar phonesxe2x80x9d are now ubiquitous, and cordless phone in the home are commonplace. A whole new batch of wireless personal digital assistants, and Bluetooth enabled computer peripherals are now entering the market, with wireless internet access as a driving force. A multi-phase mixer is described which will facilitate the design and lower the cost of circuits for these and related products.
Wireless devices typically transmit and receive data through the air on high frequency electromagnetic waveforms, though some systems, such as satellite dishes and pagers simply receive, and others merely transmit. Data transmission is begun by encoding the data to be transmitted. This encoded data typically has a data rate of 100 kHz to 100 MHz and modulates a high frequency carrier signal. The carrier signal is often in the 2-10 GHz range. The modulated carrier signal is then applied to an antenna for broadcasting. The broadcast signal is referred to as a radio frequency (RF) signal. Reception involves receiving the RF signal on a different antenna, and filtering undesired spectral components. The signal is demodulated, filtered again, and decoded.
It is very difficult to handle and generate these high frequency carrier and RF signals. Accordingly, receivers and transmitters are designed to have a minimum amount of circuitry operating at or near these rates. Transmitters are set up to modulate the carrier with the data right at the antenna. Receivers are organized to demodulate the RF to the data rate as soon in the signal path as possible.
Exacerbating this is the competitive nature of the wireless marketplace itself, which puts tremendous pricing pressure on systems manufacturers. Much of the system is on at least one integrated circuit, and that integrated circuit""s price can be reduced by producing it using a comparatively inexpensive process. For optimal savings, a process no better than what is required to make a properly functioning circuit is used. The practical aspect of this is that devices handling the carrier frequency are operating above their fbeta and near their unity gain frequency fT. In other words, the transistors in the integrated circuit have low gain and don""t operate much like transistors at these frequencies. What is needed are methods and circuits for the modulation and demodulation of carrier signals that can alleviate these difficulties at high frequency.
FIG. 1 is a block diagram of a conventional receiver 100 for use in wireless systems. Specifically, a direct conversion receiver is represented. It may also be referred to as a low IF (intermediate frequency), zero IF, or hoinodyne receiver. Included is a low noise amplifier (LNA) 110, a modulator or mixer 160, low pass filter (LPF) 120, digital signal processor (DSP) 130, voltage controlled oscillator (VCO) 140, and phase lock loop (PLL) 150. The PLL 150 includes a frequency synthesizer, phase-frequency detector, and loop filter. A variable gain amplifier (VGA) may also be included. The DSP 130 includes an analog to digital converter (A/D).
The RF signal is received on an antenna coupled to line 105. A choke filter may be used to remove unwanted spectral portions from the reception characteristics of line 105. The RF signal is amplified by LNA 110, and provided to the mixer 160. LNA 110 may be a composite of more than one amplifier, for example a second LNA may be on a chip with the other blocks shown, while a first LNA may be off-chip. A VCO 140 generates a local oscillator (LO) signal on line 145, and provides it to the mixer 160 and PLL 150. The VCO may be on-chip or off-chip; alternately it may have its transistors on-chip, with some passive components external.
The mixer 160 multiplies the RFin signal on line 115 with the LO signal on line 145. The mixer 160 outputs a signal on line IF1125, which has spectral components at the two frequencies which are the sum and difference of the RFin and LO signals. Specifically, if the RFin and LO frequencies are both 2.4 GHz, IF1125 has components at DC (0 Hz) and 4.8 GHz.
LPF 120 filters the high frequency sum products of IF1 while passing the low frequency difference components. A VGA may be used at this point to adjust the signal amplitude. The A/D converter in the DSP 130 digitizes the data, and DSP 130 decodes the data, and provides an output on line 155. The DSP 130 provides feedback in the form of a digital signal, which is converted to an analog signal for controlling the gain of the VGA. PLL 150 provides the voltage which controls the VCO 140""s oscillation frequency. The control voltage is Vtune, and is output from the PLL 150 to the VCO 140 on line 175. The PLL 150 divides the LO signal on line 145 and compares that to a reference frequency (REF) provided on line 165. The LO frequency is adjusted accordingly.
A conventional mixer circuit 200 used in similar receivers is shown in FIG. 2. The mixer has a first input port 245 labeled RFin, a second differential input port for receiving the LO signal and its complement LOS on lines 215 and 225, and a differential output on lines 265 and 275. Voltage changes at RFin generate a current in capacitor C1240. This current modulates the tail current provided by M3230 under the control of the bias voltage on node 235. This RFin modulated current is then multiplied in the mixer core M1210 and M2220, resulting in the IF1 output on the lines 265 and 275; 1he output IF1 will have two frequency components, one at the sum of the frequency of the RFin and LO signals, and one at the difference.
In the receiver of FIG. 1, the LNA 110, mixer 160, VCO 140, and frequency synthesizer portion of the PLL operate at or near the RF frequency. This leads to three difficulties. First, a large amount of the circuitry is running at high frequencies near their transistor""s fT. Second, the LO signal on line 145 induces a signal on the RF line 105, which is amplified by the LNA 110, and mixed with the LO itself in mixer 160. This is referred to as LO leakage. The result is a DC voltage which appears as a DC offset at IF1 on line 125. Third, the RF signal leaks onto the VCO, particularly at the point where external components may be connected. As the RF signal changes frequency, the VCO frequency tries to change. This is known as VCO pulling.
What is needed is a design innovation which would enable the use of comparatively inexpensive technology while still achieving the desired performance and solving the above problems. SUMMARY OF THE INVENTION
Accordingly, mixer circuits which reduce the amount of circuitry operating at or near the carrier frequency are disclosed. The mixer circuits also mitigate the LO-leakage and VCO-pulling problems.
In one embodiment, the present invention provides a wireless receiver apparatus including a VCO and mixer. The VCO provides a first signal having a first frequency, and a second signal having the first frequency. The first and second signals are in quadrature. The mixer has a first input port for receiving the first signal, and a second input port for receiving the second signal, a third input port for receiving a third signal centered about a third frequency. The mixer generates a fourth signal having a frequency centered about the third frequency less twice the first frequency.
In another embodiment, the present invention provides a mixer including a first port for receiving a first signal having a first frequency; a third port for receiving a third signal having a third frequency; and a fourth port for outputting a fourth signal having a fourth frequency. A mixer core for doubling the first frequency of the first signal and multiplying the first signal with the third signal is also included.
The mixer may also have a second port for receiving a second signal wherein the second signal is skewed relative to the first signal. In one embodiment the skew between the first and second signals is approximately 90 degrees.
In yet another embodiment, the present invention provides a method of mixing signals including providing a first signal at a first frequency, providing a second signal at the first frequency, the second signal skewed from the first, and providing a third signal modulated about a third frequency. The first signal and the second signal are exclusive-ORed, and multiplied with the third signal. The method may further include outputting a fourth signal at a frequency equal to the third frequency less twice the first frequency.